Process for the production of esters

Abstract: A process for the production of an ester or a mixture of esters, comprising allowing an olefin, carbon monoxide and an alcohol to react in the presence of hydrogen fluoride. The esters can be produced at high yields at a low temperature over a short period of time. The esters can be easily separated from the reaction solution and hydrogen fluoride can be recycled. The kinetic viscosity and the colouration of the mixture of the esters can be easily adjusted and an increase in the viscosity of the esters can be prevented.

Description: The present invention relates to a process for the production of esters for use in a variety of products such as perfumes, jet engine oils, grease for use at a low temperature, heat-resistant engine oils, refrigerating machine oils, rubbers, inks and coating compositions. More specifically, it relates to a process for the production of a mixture of esters from an olefin, carbon monoxide and an alcohol, which mixture is advantageously used as an engine oil and a refrigerating machine oil. Conventionally, an ester is produced by the reaction of dehydration in which a fatty acid and an alcohol are heated in the presence of a catalyst, as is described, e.g., US-A-5,202,044. The catalyst used for the above dehydration is selected from acid catalysts such as sulfuric acid and p-toluenesulfonic acid and organic titanate compounds such as tetrapropyl titanate. The reaction of the above dehydration is an equilibrium reaction, and an excess of an alcohol is used. The reaction is promoted by removing formed water from the reaction system. Generally, however, the reaction rate is low, and it requires a long period of time to complete the reaction. Further, for producing an ester, for example, from a branched fatty acid such as neo-acid type, it is required to proceed with the reaction at a high temperature of at least 200°C for at least 10 hours. As a result, it is required to use a large-volume reactor of a high-quality material, and it requires an additional cost. For avoiding the reaction which is carried out at a high temperature for a long period of time, JP-A-4-314793 and JP-A-5-1291 propose a method in which an ester is produced through an acid chloride. In the above method of the esterification through an acid chloride, however, it is first required to produce the acid chloride from a carboxylic acid, and for this purpose, acid chlorides of inorganic acids such as phosphorus trichloride, phosphorus pentachloride or thionyl chloride are used. These acid chlorides of inorganic acids are difficult to handle, and require complicated steps. As described above, in the conventional processes for the production of esters, the reaction rate is low and it is required to use a large-volume reactor formed of a high-quality material when they are produced by the dehydration reaction between fatty acids and alcohols. Further, in the method in which an ester is formed through an acid chloride, a complicated step is required, since acid chloride of inorganic acid is used. US-A-4590293 discloses a process for producing an ester via an acid chloride, in which waste heat is recovered from low temperature exothermic reactions. US-A-3052698 discloses a one-step synthesis for the preparation of an ester from an olefin hydrocarbon, an alcohol and carbon monoxide in the presence of hydrogen fluoride. GB-A-2101591 discloses a liquid phase carbonylation process for producing acyl halides from carbon monoxide, an anhydrous acid and an olefin. Further, esters are used in various fields as described above, and esters having performances required in various fields are selected or esters having different performances are mixed. For example, in various engine oils and refrigerating machine oils, the kinematic viscosity is particularly important, and the kinematic viscosity is adjusted by changing combinations of fatty acids and alcohols or by mixing a low-viscosity ester with a high-viscosity ester. For carrying out the esterification of various combinations, it is required to purchase or synthesize various raw materials, and it is also required to carry out the reaction under conditions suitable for the raw materials. For this reason, the reaction procedures are complicated, and it is difficult to produce various products efficiently. Further, when a low-viscosity ester and a high-viscosity ester are mixed, it is difficult in many cases to satisfy the performance and physical properties required. It is an object of the present invention to provide a process for the production of esters at high yields at a low temperature in a short period of time. It is another object of the present invention to provide a process for the production of esters, by which esters can be easily separated from a reaction solution, hydrogen fluoride can be recycled, and the esters can be produced at a low cost. It is further another object of the present invention to provide a process for the production of esters, by which a mixture of esters can be easily produced and the kinetic viscosity of the mixture of esters can be easily adjusted. It is still further another object of the present invention to provide a process for the production of esters, which permits the facile recovery of hydrogen fluoride from a reaction solution in an esterification step. It is yet another object of the present invention to provide a process for the production of esters, which permits the efficient prevention of the coloring of, and an increase in the viscosity of, the esters which are separated from a reaction solution in an esterification step and purified. According to the present invention, there is provided a process for the production of an ester or a mixture of esters by allowing an olefin, carbon monoxide and an alcohol to react in the presence of hydrogen fluoride, which process comprises: (a) reacting an olefin and carbon monoxide in the presence of hydrogen fluoride to synthesize an acyl fluoride; (b) reacting the acyl fluoride in the resultant hydrogen fluoride-containing reaction solution with an alcohol to form an ester or esters; and (c) distilling the thus-obtained esterification solution under reflux with a saturated hydrocarbon having a higher boiling point than the acyl fluoride and which is inert to the acyl fluoride, to separate and recover the hydrogen fluoride. According to the present invention, there is further provided a process wherein the olefin has at least 4 carbon atoms, and the reaction temperature and/or the reaction pressure in step (a) are/is adjusted to form esters having a desired kinetic viscosity. Also provided is a process according to the invention comprising an additional step, before step (b), of distilling the hydrogen fluoride-containing reaction solution from step (a) under reflux with a saturated hydrocarbon which has a higher boiling point than the acyl fluoride and which is inert to the acyl fluoride. The formation of byproducts in the esterification step is thus reduced. Further, there is also provided a process according to the invention further comprising: washing the ester-containing solution from step (c) with water; distilling the washed solution; and catalytically hydrogenating the distilled solution to obtain an ester or esters which have a low acid value, such as an acid value of 0.02 mgKOH/g or less, and which are decolored. In the accompanying drawings: Fig. 1 is a flow chart showing the flow of the process comprising the step of synthesizing an acyl fluoride, the step of distilling the acyl fluoride, the step of esterification and the step of recovering hydrogen fluoride, provided by the present invention Fig. 2 is a flow chart showing the flow of the process comprising the step of synthesizing an acyl fluoride, the step of esterification and the step of recovering hydrogen fluoride, provided by the present invention. Fig. 3 is a flow chart showing the flow of the process comprising the step of synthesizing an acyl fluoride, the step of esterification, the step of recovering hydrogen fluoride, the step of washing a reaction solution containing esters with water and distilling the reaction solution and the step of catalytically hydrogenating the esters. The olefin used as a raw material in the present invention is typically a C 2 -C 10 olefin, preferably a C 2 -C 8 , more preferably a C 3 -C 8 , olefin. Suitable olefins include propylene, butylene, isobutylene, pentene, hexene and octene. The alcohol used in the present invention includes monohydric C 1 -C 10 , preferably C 1 -C 8 , more preferably C 1 -C 4 , alcohols such as methanol, ethanol, propanol, isopropanol, butanol, pentanol, hexanol, heptanol and octanol, and polyhydric alcohols. Preferably the polyhydric alcohols have a functionality of 2 to 6. Preferred polyhydric alcohols are C 2 -C 10 , more preferably C 4 -C 8 alcohols such as neopentyl glycol, trimethylol ethane, trimethylol propane, pentaerythritol, ethylene glycol, propylene glycol, glycerol and sorbitol. Typically the alcohol is a monohydric alcohol or an alcohol having at least one difunctionality. The saturated hydrocarbon used in the present invention includes C 4 -C 12 hydrocarbons, and is preferably a C 6 -C 10 hydrocarbon. The hydrocarbons may be straight-chain or branched. Suitable hydrocarbons include hexane, heptane, octane, nonane and decane. Typically, in the process of the invention, the hydrogen fluoride is used in an amount of at least 5 mol, preferably 5 to 30 mol, particularly preferably 7 to 15 mol, per mole of the olefin, and the hydrogen fluoride separated and recovered in step (c) is recycled to step (a). When the molar ratio of the hydrogen fluoride to the olefin is lower than 5, the degree of occurrence of side reactions increases, and the yields of the acyl fluoride and esters are low. When the above molar ratio is greater than 30, there is no further improvement in the yields and, economically disadvantageously, a reactor having a large volume is required. In the present invention, the hydrogen fluoride may be used in combination with boron trifluoride as a catalyst. The equivalent ratio of OH group of the alcohol to the olefin is typically 2 or less, preferably 0.7 to 1.5. When the amount of the alcohol is too large, the amount of unreacted alcohol to be recovered is large. When the amount of the alcohol is too small, the yield of the esters is low. In the process of the present invention, the olefin and carbon monoxide are allowed to react in hydrogen fluoride and then the alcohol is allowed to react with the reaction product. It is assumed that, in the process of the invention, an acyl fluoride is formed by the reaction between the olefin and carbon monoxide in hydrogen fluoride and that esters and hydrogen fluoride are formed by the reaction between the acyl fluoride and the alcohol. Advantageously, esters can be produced at high yields. The pressure for the reaction between the olefin and carbon monoxide in hydrogen fluoride is generally less than 9800 kPa (100 kg/cm 2 G), preferably 980 to 7800 kPa (10 to 80 kg/cm 2 G). When this reaction pressure is low, the yield of the acyl fluoride is low. However, even when the reaction pressure is made higher than 9800 kPa (100kg/cm 2 G), there is almost no improvement in the yield and, industrially disadvantageously, the cost of an apparatus therefor increases. The amount of the carbon monoxide used for the reaction between the olefin and the carbon monoxide is determined depending upon the reaction pressure. The above-specified reaction pressure is a reaction pressure when high-purity carbon monoxide is used. When the gas of the carbon monoxide contains a considerable amount of inert gas, the partial pressure of carbon monoxide is to be taken as the reaction pressure. Although differing depending upon the olefin, the temperature required for the reaction between the olefin and carbon monoxide is generally -30 to 70°C, preferably -20 to 50°C. When the reaction temperature is too high, the amount of byproducts increases and the yield of the acyl fluoride decreases. When the reaction temperature is too low, economically disadvantageously, the reaction rate is low. The reaction of the product (acyl fluoride) from the olefin and carbon monoxide with the alcohol may be carried out under elevated pressure or atmospheric pressure. That is, the olefin and carbon monoxide are allowed to react under elevated pressure, and the reaction product may be allowed to react with the alcohol under the so-elevated pressure. Otherwise, the reaction product may be allowed to react with the alcohol after the pressure is reduced. The temperature for the reaction between the above reaction product and the alcohol is generally -10 to 70°C, preferably 0 to 50°C. When the olefin and carbon monoxide are allowed to react in the presence of hydrogen fluoride, some olefins undergo isomerization, and the ester obtained by the esterification is an isomer mixture. The isomer mixture differs in composition depending upon the reaction temperature and pressure employed for the synthesis of the acyl fluoride, and proper reaction conditions are selected. When an engine oil or a refrigerating machine oil is produced according to the process of the present invention, the olefin used as a raw material in the step of synthesizing acyl fluoride is selected from olefins having at least 4 carbon atoms, and the reaction temperature or pressure is changed, whereby there can be obtained esters having a desired kinetic viscosity. In general, with an increase in the reaction temperature In the step of synthesizing acyl fluoride, the kinetic viscosity of a mixture of esters obtained in the esterification step increases. Further, with an increase in the reaction pressure in the step of synthesizing acyl fluoride, the viscosity of a mixture of esters obtained in the esterification step decreases. The reaction solution obtained in the process of the present invention is a solution of esters in hydrogen fluoride, and the process of the present invention hence does not particularly require any solvent. However, there may be used a solvent which does not affect the reaction. For example, there may be used a saturated hydrocarbon such as hexane, heptane or octane. The solvent may be used, for example, by a method in which the olefin is dissolved in the solvent and the solution is fed to a reactor. Esters can be obtained by the process of the invention at high yields at a low temperature in a short period of time. However, some olefins and some alcohols form various byproducts due to the polymerization of the olefins or reactions of the olefins, carbon monoxide and alcohols. As a result, in some cases, the yield in the esterification is limited, and many distillation columns are required for separating the intended esters. In the process of the invention, therefore, the reaction solution obtained from step (a) may be distilled under reflux with a saturated hydrocarbon having a higher boiling point than the acyl fluoride and which is inert to the acyl fluoride, whereby the formation of byproducts in the esterification step decreases. As a result, the time required for operating the esterification step is decreased, and the yield in the esterification improves. Further, the separation and purification of the esters are facilitated. The above saturated hydrocarbon is typically selected from hexane, heptane, octane, nonane and decane. In this distillation step, the reaction solution from step (a) is generally distilled under reflux with the saturated hydrocarbon in a distillation column, and the saturated hydrocarbon is typically fed through an inlet provided at a lower position than that for the reaction solution containing the acyl fluoride. Hydrogen fluoride and high-purity acyl fluoride may be separated through the distillation column top and introduced to the esterification step. A mixture of saturated hydrocarbon with byproducts such as polymers of olefin formed in the acyl fluoride synthesis step may be separated through the distillation column bottom. The saturated hydrocarbon can easily be separated from the column bottom solution by distillation, and recycled. Typically, the saturated hydrocarbon used in the additional step of distilling the reaction solution obtained from step (a), is used in an amount of 0.5 to 2.0 mol, preferably 1.0 to 1.5 mol, per mole of the olefin. When the molar ratio of the saturated hydrocarbon to the olefin is less than 0.5, it is difficult to obtain a high-purity acyl fluoride from the top of the distillation column. When the above molar ratio is greater than 2.0, uneconomically, the amount of heating energy is large, and the amount of the saturated hydrocarbon to be recovered is large. The temperature for the distillation of the acyl fluoride differs depending upon the olefin and the saturated hydrocarbon used, while the temperature in the bottom of the distillation column is generally 100 to 250°C, preferably 120 to 200°C. The distillation is advantageously carried out under atmospheric pressure or elevated pressure, while the pressure for the distillation is preferably 50 to 100 kPa (0.5 to 1.0 kg/cm 2 G). The hydrogen fluoride is easily separated by distilling the esterification reaction solution obtained by the reaction of the acyl fluoride and alcohol. The above-separated hydrogen fluoride can be recycled. Some esters can be recovered by pouring the esterification reaction solution into ice water and distilling a separated oil layer. The esterification reaction solution is distilled with the distillation column (to be referred to as "HF-recovery distillation column" hereinafter) under the reflux of the saturated hydrocarbon, whereby the hydrogen fluoride is easily separated, and it can be recycled. When unreacted acyl fluoride is present, a saturated hydrocarbon which has a higher boiling point than the acyl fluoride and is inert to the acyl fluoride is used for recovering both this acyl fluoride and hydrogen fluoride from the column top. The above saturated hydrocarbon is selected from hexane, heptane octane, nonane and decane. The reaction solution and saturated hydrocarbon can be recovered from the bottom of the HF-recovery distillation column. The amount of the saturated hydrocarbon used in the HF-recovery distillation column per mole of the olefin is generally 0.5 to 2.0 mol, preferably 1.0 to 1.5 mol. When the molar ratio of the saturated hydrocarbon to the olefin is less than 0.5, it is difficult to a sufficient amount of HF from the column top. When the above molar ratio is greater than 2.0, uneconomically, the amount of heating energy is large and the amount of saturated hydrocarbon to be recovered is large. The temperature for the distillation in the HF-recovery distillation column differs depending upon the olefin and saturated hydrocarbon used, while the temperature in the bottom of the HF-recovery distillation column is generally 100 to 250°C, preferably 120 to 200°C. The distillation is processwise advantageously carried out under atmospheric pressure or elevated pressure, while the pressure for the distillation is preferably 50 to 100 kPa (0.5 to 1.0 kg/cm 2 G). In the above HF-recovery distillation column, it is preferred to feed the saturated hydrocarbon through a portion lower than a portion through which the esterification reaction solution is fed. The hydrogen fluoride is separated through the column top, and recycled to the acyl fluoride synthesis step. The esters, byproducts and the saturated hydrocarbon are separated through the column bottom. The saturated hydrocarbon is easily separated from the column bottom solution by distillation, and recycled to the HF-recovery distillation column. In the present invention, the hydrogen fluoride is first recovered from the resultant reaction solution by the distillation under the reflux of the saturated hydrocarbon, and then the reaction solution can be washed with water, purified by distillation and catalytic hydrogenation treatment, whereby the resultant ester(s) has an acid value of 0.02 mgKOH/g or less and is decolored. Therefore, ester(s) for use as an engine oil and a refrigerating machine oil can be very advantageously produced. That is, after the hydrogen fluoride is recovered from the reaction solution, the reaction solution is purified by distillation and catalytically hydrogenation-treated. In the purification step, the column bottom solution from the HF-recovery distillation column is washed with water and subjected to distillation. The saturated hydrocarbon used in the HF-recovery distillation column is easily separated and recovered by simple distillation, and the recovered saturated hydrocarbon is recycled. Then, the reaction solution is purified and distilled to remove byproducts, whereby the ester(s) is obtained. In the distillation for recovering the hydrogen fluoride, the ester(s) is colored. Further, due to the heating during the distillation of the ester, organic acids are formed, and the ester(s) has an increased acid value. The ester(s) which is colored and has an increased acid value is not suitable for use as a refrigerating machine oil and an engine oil. This problem can be overcome by catalytically hydrogenation-treating the reaction solution. The distillation temperature for the recovery of the saturated hydrocarbon differs depending upon the saturated hydrocarbon used. The distillation temperature in the column bottom is generally 50 to 150°C, preferably 60 to 120°C. The distillation for the recovery of the saturated hydrocarbon is processwise advantageously carried out under reduced pressure, and the pressure for the distillation is preferably 1330 to 8000 Pa (10 to 60 torr). The distillation temperature for obtaining the ester(s) differs depending upon the ester(s) synthesized. The distillation temperature in the column bottom is generally 100 to 300°C, preferably 150 to 280°C. The distillation for obtaining the ester(s) is processwise advantageously carried out under reduced pressure, and the pressure for the distillation is preferably 400 to 2700 Pa (3 to 20 torr). The catalytic hydrogenation treatment is typically carried out in the presence of a hydrogenating catalyst such as an Ru-, Pd- or Ni-containing catalyst. The amount of the catalyst per part of the ester(s) to be treated is 0.01 to 0.30 part by weight, preferably 0.04 to 0.20 part by weight. When the catalyst amount is smaller than the above lower limit, the hydrogenation treatment is not sufficient. When the catalyst amount is larger than the above upper limit, no further improvement is exhibited in the hydrogenation treatment, and industrially disadvantageously a reactor having a large volume is required. Although differing depending upon the catalyst used, the temperature for the catalytic hydrogenation treatment is generally 70 to 110°C, preferably 80 to 100°C. When the above temperature is too low, the decoloration of the ester(s) is insufficient, and the acid value does not decrease. When the temperature is too high, the ester(s) is colorless and transparent, but the acid value increases. Although differing depending upon the catalyst used, the pressure for the catalytic hydrogenation treatment is generally 490 to 1470 kPa (5 to 15 kg/cm 2 /G), preferably 690 to 980 kPa (7 to 10 kg/cm 2 ). When the above pressure is too low, the treatment rate is low and the decoloration is not sufficient. Further, even when the pressure is made higher than 1470 kPa (15 kg/cm 2 G), there is no improvement in acid value, and industrially disadvantageously, an additional cost is required for an apparatus. The present invention will be explained hereinafter with reference to drawings. Fig. 1 shows the flow chart of the process for the production of esters, provided by the present invention, the process comprising (i) an acyl fluoride synthesis step in which the olefin and carbon monoxide are allowed to react in the presence of hydrogen fluoride, (ii) an acyl fluoride distillation step in which the reaction solution containing the formed acyl fluoride is distilled together with the saturated hydrocarbon having a higher boiling point than the acyl fluoride and being inert to the acyl fluoride, and (iii) an esterification step in which a mixture of the hydrogen fluoride and acyl fluoride, separated in the distillation step is allowed to react with the alcohol. In the acyl fluoride synthesis step shown in Fig. 1, hydrogen fluoride is fed from a flow path 1 to an acyl fluoride synthesis reactor 4, and carbon monoxide is charged into the reactor through a flow path 3 up to a predetermined pressure. Then, an olefin is fed through a flow path 2 to synthesize an acyl fluoride. In the next acyl fluoride distillation step, the reaction solution containing the acyl fluoride is introduced into an acyl fluoride distillation column 7 through a flow path 5, and the saturated hydrocarbon is charged into the acyl fluoride distillation column 7 through a flow path 6. The distillation column 7 includes a packed column and a plate column. When the packed column is used, it is divided into an upper packed portion 8, an intermediate packed portion 9 and a lower packed portion 10 for increasing the distillation efficiency. And, the reaction mixture containing the acyl fluoride is fed into a portion between the upper packed portion 8 and the intermediate packed portion 9, and the saturated hydrocarbon is fed into a portion between the intermediate packed portion 9 and the lower packed portion 10. In the distillation column, the heating is effected with a reboiler 11, whereby steams of HF, acyl fluoride and saturated hydrocarbon are obtained from the top of the distillation column, totally condensed in a condenser 13 and introduced into a separator 14. In the separator 14, there is a complex of HF and acyl fluoride, and the steams are separated into two layers, a layer of the complex and a layer of the saturated hydrocarbon, due to a specific gravity difference between the complex and the saturated hydrocarbon. The saturated hydrocarbon in the upper layer portion is drawn through a flow path 15 and recycled to the distillation column. HF and the acyl fluoride are drawn through a flow path 16, and introduced into an esterification reactor 18. In the esterification step, the HF and the acyl fluoride are fed into the esterification reactor 18 through the flow path 16, and the alcohol is fed into the esterification reactor 18 through a flow path 17, to carry out the esterification. After the esterification, the esterification reaction solution is introduced into a catalyst recovery distillation column 20 through a flow path 19. HF is recovered from a column top 21, and esters are obtained from a column bottom 22. In another method, the esterification reaction solution is poured into ice water through a flow path 23, and esters can be separated from an oil layer. Fig. 2 shows the flow chart of the process of the present invention, which comprises allowing the olefin and carbon monoxide in the presence of hydrogen fluoride to form an acyl fluoride, allowing the acyl fluoride and the alcohol to react to form esters, and distilling the reaction solution containing the esters under the reflux of a saturated hydrocarbon having a higher boiling point than the acyl fluoride and being inner to the acyl fluoride to decompose the reaction solution into the esters and hydrogen fluoride. In the acyl fluoride synthesis step shown in Fig. 2, hydrogen fluoride is fed into an acyl fluoride reactor 34 through a flow path 31 and carbon monoxide is charged into the acyl fluoride reactor 34 through a flow path 33 up to a predetermined pressure. Then, the olefin is fed through a flow path 32 to synthesize an acyl fluoride. The reaction solution containing the acyl fluoride is introduced into the esterification step. In the esterification step, the reaction solution containing the acyl fluoride is fed into an esterification reactor 37 through a flow path 35, and the alcohol is fed into the esterification reactor 37 through a flow path 36, to carry out the esterification. After the reaction, the esterification reaction solution is introduced into an HF-recovery distillation column 40 through a flow path 38, and the saturated hydrocarbon is introduced into the HF-recovery distillation column 40 through a flow path 39. The HF-recovery distillation column 40 includes a packed column and a plate column. When a packed column is used, it is divided into an upper packed portion 41, an intermediate packed portion 42 and a lower packed portion 43 for increasing the distillation efficiency. And, the esterification reaction solution is fed into a portion between the upper packed portion 41 and the intermediate packed portion 42, and the saturated hydrocarbon is fed into a portion between the intermediate packed portion 42 and the lower packed portion 43. In the HF-recovery distillation column, the heating is effected with a reboiler 44, whereby steams of HF and saturated hydrocarbon are obtained from the top of the distillation column, totally condensed in a condenser 46 and introduced into a separator 47. In the separator 47, the hydrogen fluoride forms one layer, and the saturated hydrocarbon forms another layer, due to a specific gravity difference between the hydrogen fluoride and the saturated hydrocarbon. The saturated hydrocarbon in the upper layer portion is drawn through a flow path 48 and recycled to the distillation column. The hydrogen fluoride is drawn through a flow path 49 and recycled. Esters, byproducts and saturated hydrocarbon are drawn through a flow path 45 connected to the column bottom. Fig. 3 is the flow chart of the process for the production of esters, provided by the present invention, which comprises allowing the olefin, carbon monoxide and the alcohol to react in the presence of hydrogen fluoride, distilling the resultant reaction solution under the reflux of the saturated hydrocarbon to recover hydrogen fluoride, washing the reaction solution with water, distilling and purifying the reaction solution, and carrying out the catalytic hydrogenation treatment. In Fig. 3, hydrogen fluoride is fed into a reactor 55 through a flow path 51 and carbon monoxide is charged into the reactor 55 through a flow path 53 up to a predetermined pressure. Then, the olefin is fed through a flow path 52 to synthesize an acyl fluoride. After the synthesis is completed, the pressure in the reactor is reduced to atmospheric pressure, and then, the alcohol is fed through a flow path 54 to synthesize an ester. After the ester(s) is synthesized, the reaction solution and hydrogen fluoride are introduced into an HF-recovery distillation column 58 through a flow path 56, and the saturated hydrocarbon is introduced into the HF-recovery distillation column through a flow path 57. Hydrogen fluoride and saturated hydrocarbon are obtained from the top of the HF-recovery distillation column 58, and the saturated hydrocarbon is recycled to the HF-recovery distillation column 58 through a flow path 59. The hydrogen fluoride is drawn through a flow path 60, and recycled. The reaction solution and saturated hydrocarbon are drawn through a flow path 61 connected to the bottom of the distillation column, and introduced into a washing vessel 64. Washing water is introduced into the washing vessel through a flow path 62 to wash the reaction solution, and the reaction solution is allowed to stand. Then, the washing water is drawn through a flow path 63. The washed reaction solution and saturated hydrocarbon are drawn through a flow path 65. In a purification distillation column (I) 66, first, saturated hydrocarbon having a low boiling point is distilled off through a flow path 67, and recycled. The esters and components having a high boiling point from the column bottom are introduced into a purification distillation column (II) 69 through a flow path 68. In the purification distillation column (II), the ester(s) is distilled out through the column top, and introduced into a hydrogenation treatment vessel 64 through a flow path 70. Components having a high boiling point are drawn through a flow path 71 connected to the bottom of the purification distillation column (II). In the hydrogenation treatment vessel 74, the ester fraction introduced through the flow path 70 is catalytically hydrogenation-treated in the presence of a catalyst introduced through a flow path 72 and hydrogen introduced through a flow path 73. The esters and catalysts are drawn through a flow path 75, and filtered to separate them. In the process of the present invention, esters can be obtained from an olefin, carbon monoxide and an alcohol at high yields by the reaction thereof at a low temperature for a short period of time. Further, in the process of the present invention, the reaction rate is high so that the reactor therefor can be decreased in size, and hydrogen fluoride used as a catalyst can be easily separated from the reaction product and recycled. Therefore, esters can be highly advantageously produced from the industrial viewpoint. Further, in the process of the present invention, the step of producing esters is divided into the acyl fluoride synthesis step in which the olefin and carbon monoxide are allowed to react in the presence of hydrogen fluoride and the esterification step in which the acyl fluoride synthesis solution is allowed to react with the alcohol, and in the acyl fluoride synthesis step, an olefin having at least 4 carbon atoms can be used as the olefin and the reaction temperature or pressure can be changed, whereby the kinetic viscosity of a mixture of esters can be adjusted, and there can be obtained a mixture of esters having a kinetic viscosity required as an engine oil or a refrigerating machine oil. That is, in the process of the present invention, it is not necessary to change the combinations of fatty acids and alcohols or mix a low-viscosity ester and a high-viscosity ester for the adjustment of the kinetic viscosity, and mixtures of esters for use as various engine oils and refrigerating machine oil can be industrially advantageously produced. In the present invention, the process for the production of esters is divided into the acyl fluoride synthesis step in which the olefin and carbon monoxide are allowed to react in the presence of hydrogen fluoride and the esterification step in which the acyl fluoride obtained is allowed to react with the alcohol, and the reaction solution from the acyl fluoride synthesis step can be distilled under the reflux of a saturated hydrocarbon which has a higher boiling point than the acyl fluoride and is inert to the acyl fluoride. As a result, industrially advantageously, the following effects are obtained. a) Since the next step of esterification can be carried out at a low temperature for a short period of time, the amount of byproducts is small, and the yield of the esters improves. b) Since the amount of byproducts from the esterification is small, the purification can be easily carried out, and high-purity esters can be obtained. c) The amount of energy used for heating the reaction solution for recovering hydrogen fluoride as a catalyst can be reduced. Further, in the process of the present invention, the olefin and carbon monoxide are allowed to react in the presence of hydrogen fluoride to form an acyl fluoride, the acyl fluoride is allowed to react with the alcohol to produce esters, and a complex of the ester(s) and hydrogen fluoride is decomposed by distilling the reaction solution under the reflux of a saturated hydrocarbon which has a higher boiling point than the acyl fluoride and is inert to the acyl fluoride, whereby the hydrogen fluoride can be efficiently recycled, and the esters can be advantageously produced. Moreover, in the present invention, the reaction solution from which the hydrogen fluoride has been recovered can be purified by washing it with water and distillation, and the purified reaction solution is catalytically hydrogenation-treated, whereby the ester(s) has an acid value of 0.02 mgKOH/g or less and is decolored. According to the process of the present invention, therefore, ester(s) for use as an engine oil or a refrigerating machine oil can be advantageously produced. The present invention will be further detailed hereinafter with reference to Examples. However, the present invention shall not be limited to these Examples. In Examples, the acid value was analyzed by a titration method using an N/25 potassium hydroxide solution, the hydroxyl value was analyzed by a pyridine-acetic anhydride method, the kinetic viscosity was measured with an Ubbelohde viscometer, and the color of an ester was measured with an ASTM color tester. Reference Example 1 Esters were produced with a two-liter autoclave of stainless steel, having a stirrer, three inlet nozzles provided in an upper portion, one outlet nozzle provided in a bottom portion and a jacket for adjusting the temperature inside. First, the above autoclave was flushed with carbon monoxide, and then 420 g (21 mol) of hydrogen fluoride was introduced. The hydrogen fluoride was cooled to -10°C, and the pressure inside the autoclave was increased up to 1960 kPa (20 kg/cm 2 ) with carbon monoxide. While this reaction temperature was maintained and while carbon monoxide was introduced so as to maintain the above pressure, 160 g (1.9 mol) of 1-hexene was fed through a gaseous phase in the autoclave over about 60 minutes, and after the feeding finished, the mixture was continuously stirred for about 20 minutes. The amount of carbon monoxide absorbed by the reaction was 39.2 g (1.4 mol), which was calculated on the basis of a weight difference in a carbon monoxide measuring vessel. Then, the pressure inside the autoclave was reduced to atmospheric pressure to purge unreacted carbon monoxide, and 48 g (1.5 mol) of methanol was fed into the autoclave with a pump over about 30 minutes. The reaction temperature was set at 10°C, and the mixture was stirred for 2 hours. After the reaction, the reaction solution was poured into ice water to separate an oil layer. The obtained oil layer was washed with water and distilled at 2.6 kPa (20 mmHg) to give 197 g of a fraction having a boiling point of 50 to 53°C. This fraction was analyzed by gas chromatography to show that the fraction was an isomer mixture containing 25 % by weight of 2,2-dimethylpentanoic acid methyl ester, 8 % by weight of 2-ethyl-2-methylbutanoic acid methyl ester, 47 % by weight of 2-ethylpentanoic acid methyl ester and 20 % by weight of 2-methylhexanoic acid methyl ester. The yield of the esters based on the equivalent of the fed 1-hexene was 72 mol%. Example 1 The esterification was carried out in the same manner as in Reference Example 1 except that the methanol was replaced with n-propanol, and then, the resultant solution of esters in hydrogen fluoride was distilled through an HF-recovery distillation column (inner diameter 55 mm, height 1,000 mm, packed with Teflon packing ring) at a pressure of 200 kPa (2 kg/cm 2 G) at a column bottom temperature of 120°C under the reflux of hexane to recover hydrogen fluoride. A hexane solution containing esters and an excess of propanol was drawn from the column bottom, distilled and purified to give 231 g of an isomer mixture containing four n-propyl esters, i.e., 25 % by weight of 2,2-dimethylpentanoic acid propyl ester, 8 % by weight of 2-ethyl-2-methyl-butanoic acid propyl ester, 47 % by weight of 2-ethylpentanoic acid propyl ester and 20 % by weight of 2-methylhexanoic acid propyl ester. The yield of the esters based on the equivalent of the fed 1-hexene was 71 mol%. Example 2 The esterification was carried out in the same manner as in Example 1 except that the n-propanol was replaced with trimethylolpropane, and hydrogen fluoride was recovered from the resultant hydrogen fluoride solution of esters through the same HF-recovery distillation column as that used in Example 1. The column bottom solution was simple-distilled to give a 208 g fraction having a boiling point, at 260 to 400 Pa (2 - 3 mmHg), of 220 to 230°C, and this fraction was analyzed by gas chromatography to show an isomer mixture having a triester purity of at least 99 %. The yield of the esters on the basis of the fed 1-hexene was 70 mol%. This isomer mixture had the following physical properties, a kinetic viscosity of 23.6 cst (40°C), a specific gravity of 0.954 (20/20°C) and a hydroxyl value of not more than 5 mgKOH/g. Example 3 Esters were produced with two one-liter autoclaves of stainless steel, having a stirrer, three inlet nozzles provided in an upper portion, one outlet nozzle provided in a bottom portion and a jacket for adjusting the temperature inside, as a reactor for the synthesis of an acyl fluoride and a reactor for the esterification and one distillation column of stainless steel, which had a column diameter of 30 mm and a height of 1,000 mm and was packed with 12 mm (1/2 inch) Teflon packing ring, as an HF-recovery distillation column. First, the reactor for acyl fluoride synthesis was flushed with carbon monoxide, and then 210 g (10.5 mol) of hydrogen fluoride was introduced. The hydrogen fluoride was cooled to -15°C, and the pressure inside the autoclave was increased up to 2000 kPa (20 kg/cm 2 ) with carbon monoxide. While this reaction temperature was maintained and while carbon monoxide was introduced so as to maintain the above pressure, 91 g (0.81 mol) of 1-octene was fed through a gaseous phase in the autoclave over about 30 minutes, and after the feeding finished, the mixture was continuously stirred for about 20 minutes until no absorption of carbon monoxide was observed. The amount of carbon monoxide absorbed by the reaction was 17.6 g (0.63 mol), which was calculated on the basis of a weight difference in a carbon monoxide measuring vessel. Then, the above-obtained reaction solution containing an acyl fluoride and 32.8 g (0.315 mol) of neopentyl glycol were fed to the reactor for the esterification at atmospheric pressure at 0°C over about 20 minutes, and the mixture was allowed to react for 2 hours. After the reaction finished, the resultant esterification reaction solution and n-decane were introduced to the HF-recovery distillation column, and the reaction solution was distilled under the reflux of the n-decane at a pressure of 50 kPa (0.5 kg/cm 2 G) at a column bottom temperature of 200°C. And, the hydrogen fluoride was recovered from the column top. The so-recovered hydrogen fluoride was analyzed to show that it had a purity of 99.5 % by weight and contained 0.5 % by weight of dissolved n-decane. The amount of the recovered hydrogen fluoride was 208.9 g, and the recovery ratio thereof was 99.5 %. Esters and n-decane were drawn from the column bottom and washed with water. The n-decane was removed by simple distillation, and the remainder was further rectified. The main fraction was analyzed by gas chromatography to show that it was 115.2 g of an isomer mixture having an ester purity of at least 99 %. The yield of the esters on the basis of the fed 1-octene was 74.0 mol%. The above-obtained isomer mixture had the following physical properties, a kinetic viscosity of 10 cst (40°C) and a hydroxyl value of not more than 2 mgKOH/g. Example 4 An acyl fluoride was synthesized in the same manner as in Example 3 except that the 1-octene was replaced with 68.0 g (0.81 mol) of 1-hexene. Then, 31.1 g (0.30 mol) of neopentyl glycol was fed to carry out the esterification. After the esterification finished, the esterification reaction solution and n-nonane were introduced into an HF-recovery distillation column, and the esterification reaction solution was distilled under the reflux of the n-nonane at a pressure of 50 kPa (0.5 kg/cm 2 G) at a column bottom temperature of 175°C. And, hydrogen fluoride was recovered from the column top. The recovered hydrogen fluoride was analyzed to show it had a purity of 99.3 % by weight, and contained 0.7 % by weight of n-nonane. The amount of the recovered hydrogen fluoride was 208.7 g, and the recovery ratio thereof was 99.4 %. A solution of esters and n-nonane was drawn from the column bottom, and washed with water. The n-nonane was removed by simple distillation, and the remainder was rectified. The main fraction was analyzed by gas chromatography to show that it was an isomer mixture having an ester purity of at least 99 %. The yield of the esters was 92.8 g. The yield of the esters on the basis of the fed 1-hexene was 70.0 mol%. The above-obtained isomer mixture had the following physical properties, a kinetic viscosity of 6 cst (40°C) and a hydroxyl value of not more than 2 mgKOH/g. Example 5 An acyl fluoride was synthesized at -10°C in the same manner as in Example 3 except that the 1-octene was replaced with 45.4 g (0.81 mol) of 1-butene. 26.8 Grams (0.20 mol) of trimethylolpropane was introduced into the esterification reactor with a pump at atmospheric pressure at 0°C over about 15 minutes, and the mixture was allowed to react for 2 hours. The esterification reaction solution was distilled through the same HF-recovery distillation column as that used in Example 3 under the reflux of n-heptane at a pressure of 100 kPa (1.0 kg/cm 2 G) at a column bottom temperature of 130°C, and hydrogen fluoride was recovered from the column top. The recovered hydrogen fluoride was analyzed to show it had a purity of 99.3 % by weight, and contained 0.7 % by weight of n-heptane. The amount of the recovered hydrogen fluoride was 208.3 g, and the recovery ratio thereof was 99.2 %. A solution of esters and n-heptane was drawn from the column bottom, and washed with water. The n-heptane was removed by simple distillation, and the remainder was rectified to distill out a triester. The triester was analyzed by gas chromatography to show that it was an isomer mixture having a triester purity of at least 99 %. The yield of the isomer mixture was 73.3 g. The yield of the esters on the basis of the fed 1-butene was 70.4 mol%. The above-obtained isomer mixture had the following physical properties, a kinetic viscosity of 13 cst (40°C) and a hydroxyl value of not more than 2 mgKOH/g. Example 6 The esterification was carried out in the same manner as in Example 3 except that the neopentyl glycol was replaced with ethanol. The esterification reaction solution was distilled through an HF-recovery distillation column under the reflux of n-decane. Hydrogen fluoride was recovered from the column top, and esters and n-decane were recovered from the column bottom and washed. The n-decane was removed by distillation, and the remainder was rectified to give 110 g of an oil. The recovered hydrogen fluoride was analyzed to show it had a purity of 99.3 % by weight, and contained 0.7 % by weight of n-decane. The amount of the recovered hydrogen fluoride was 208.3 g, and the recovery ratio thereof was 99.2 %. The above-obtained oil was analyzed by gas chromatography to show that it was an isomer mixture containing six esters such as 9 % by weight of 2-methyl-2-propylpentanoic acid ethyl ester, 14 % by weight of 2,2-dimethylheptanoic acid ethyl ester, 19 % by weight of 2-methyl-2-ethylhexanoic acid ethyl ester, 24 % by weight of 2-propylhexanoic acid ethyl ester, 26 % by weight of 2-ethylheptanoic acid ethyl ester and 8 % by weight of 2-methyloctanoic acid methyl ester. The yield of the esters on the basis of the fed 1-octene was 73.0 mol%. Example 7 Esters were produced with two one-liter autoclaves of stainless steel, having a stirrer, three inlet nozzles provided in an upper portion, one outlet nozzle provided in a bottom portion and a jacket for adjusting the temperature inside, one distillation column (HF-recovery distillation column) of stainless steel, which had a column diameter of 30 mm and a height of 1,000 mm and was packed with 12 mm (1/2 inch) Teflon packing ring, and one 0.5-liter distillation column (for ester distillation) of glass. First, one autoclave was flushed with carbon monoxide, and then 210 g (10.5 mol) of hydrogen fluoride was introduced. The hydrogen fluoride was cooled to -18°C, and the pressure inside the autoclave was increased up to 2000 kPa (20 kg/cm 2 ) with carbon monoxide. While this reaction temperature was maintained and while carbon monoxide was introduced so as to maintain the above pressure, 91 g (0.81 mol) of 1-octene was fed through a gaseous phase in the autoclave over about 30 minutes, and after the feeding finished, the mixture was continuously stirred for about 20 minutes until no absorption of carbon monoxide was observed. The amount of carbon monoxide absorbed by the reaction was 17.6 g (0.63 mol), which was calculated on the basis of a weight difference in a carbon monoxide measuring vessel. Then, the pressure inside the autoclave was reduced to atmospheric pressure, and 21.5 g (0.158 mol) of pentaerythritol was fed into the autoclave over about 30 minutes. The reaction temperature was set at 0°C, and the mixture was stirred for 2 hours. After the reaction finished, the resultant reaction solution and n-decane were introduced to the HF-recovery distillation column, and the reaction solution was distilled under the reflux of the n-decane at a pressure of 50 kPa (0.5 kg/cm 2 G) at a column bottom temperature of 200°C. And, hydrogen fluoride was recovered from the column top, and a solution of esters and n-decane was drawn from the column bottom and washed. The n-decane was removed by simple distillation, and the remainder was rectified to give 103.5 g of a yellow ester fraction. The ester fraction was measured for an acid value to show 0.055 mgKOH/g. The above-obtained ester and 17.3 g of a 5 % Ru/C catalyst (supplied by N. E. CHEMCAT Corporation) were charged into the autoclave, and the mixture was stirred at a temperature of 80°C at a hydrogen pressure of 780 kPa (8 kg/cm 2 G) for 2 hours. The reaction solution was cooled to room temperature, and the catalyst was separated by filtration to give 100.5 g of a colorless transparent liquid. This liquid was analyzed by gas chromatography to show that it was an isomer mixture having a tetraester purity of at least 99 %. The yield thereof on the basis of the fed 1-octene was 71.3 mol%. The obtained tetraester had an acid value of 0.012 mgKOH/g, a kinetic viscosity of 62 cst (40°C), a hydroxyl value of not more than 2 mgKOH/g and a color, measured with an ASTM color tester, of L0.5. Example 8 An ester was produced in the same manner as in Example 7 except that pentaerythritol was replaced with 32.9 g (0.316 mol) of neopentyl glycol. The esterification reaction solution was rectified to give 114.6 g of a yellow ester fraction. This fraction had an acid value of 0.058 mgKOH/g. The above-obtained ester fraction and 19.1 g of an Ni/diatomaceous earth catalyst [N103, Ni about 50 %, supplied by Nikki Chemical Co., Ltd.) were charged into the autoclave, and the mixture was stirred at a temperature of 100°C at a hydrogen pressure of 780 kPa (8 kg/cm 2 G) for 2 hours. The reaction solution was cooled to room temperature, and the catalyst was separated by filtration to give 113.4 g of a colorless transparent liquid. This liquid was analyzed by gas chromatography to show that it was an isomer mixture having a diester purity of at least 99 %. The yield thereof on the basis of the fed 1-octene was 72.8 mol%. The obtained diester had an acid value of 0.013 mgKOH/g, a kinetic viscosity of 10 cst (40°C), a hydroxyl value of not more than 2 mgKOH/g and a color, measured with an ASTM color tester, of L0.5. Example 9 An ester was produced in the same manner as in Example 7 except that the 1-octene was replaced with 45.4 g (0.81 mol) of 1-butene, that the temperature for the synthesis of an acyl fluoride was set at -13°C and that the pentaerythritol was replaced with 26.8 g (0.20 mol) of trimethylolpropane. The esterification reaction solution was rectified to give 73.3 g of a yellow ester fraction. This fraction had an acid value of 0.045 mgKOH/g. The above-obtained ester fraction and 12.2 g of a 5 % Pd/Cu catalyst (supplied by N. E. CHEMCAT Corporation) were charged into the autoclave, and the mixture was stirred at a temperature of 90°C at a hydrogen pressure of 780 kPa (8 kg/cm 2 G) for 2 hours. The reaction solution was cooled to room temperature, and the catalyst was separated by filtration to give 71.3 g of a colorless transparent liquid. This liquid was analyzed by gas chromatography to show that it was an isomer mixture having a triester purity of at least 99 %. The yield thereof on the basis of the fed 1-butene was 68.5 mol%. The obtained triester had an acid value of 0.010 mgKOH/g, a kinetic viscosity of 13 cst (40°C), a hydroxyl value of not more than 2 mgKOH/g and a color, measured with an ASTM color tester, of L0.5. Example 10 An ester was produced in the same manner as in Example 7 except that the 1-octene was replaced with 68.0 g (0.81 mol) of 1-hexene, that the temperature for the synthesis of an acyl fluoride was set at -13°C and that the pentaerythritol was replaced with 26.8 g (0.20 mol) of trimethylolpropane. The esterification reaction solution was rectified to give 90.1 g of a yellow ester fraction. This fraction had an acid value of 0.047 mgKOH/g. The above-obtained ester fraction and 15.0 g of a 2 % Ru/Al catalyst (supplied by N. E. CHEMCAT Corporation) were charged into the autoclave, and the mixture was stirred at a temperature of 80°C at a hydrogen pressure of 780 kPa (8 kg/cm 2 G) for 2 hours. The reaction solution was cooled to room temperature, and the catalyst was separated by filtration to give 89.2 g of a colorless transparent liquid. This liquid was analyzed by gas chromatography to show that it was an isomer mixture having a triester purity of at least 99 %. The yield thereof on the basis of the fed 1-hexene was 70.4 mol%. The obtained triester had an acid value of 0.013 mgKOH/g, a kinetic viscosity of 22 cst (40°C), a hydroxyl value of not more than 2 mgKOH/g and a color, measured with an ASTM color tester, of L0.5.

Claims: A process for the production of an ester or a mixture of esters by allowing an olefin, carbon monoxide and an alcohol to react in the presence of hydrogen fluoride, which process comprises: (a) reacting an olefin and carbon monoxide in the presence of hydrogen fluoride to synthesize an acyl fluoride; (b) reacting the acyl fluoride in the resultant hydrogen fluoride-containing reaction solution with an alcohol to form an ester or esters; and (c) distilling the thus-obtained esterification solution under reflux with a saturated hydrocarbon having a higher boiling point than the acyl fluoride and which is inert to the acyl fluoride, to separate and recover the hydrogen fluoride. A process according to claim 1, wherein the hydrogen fluoride is used in an amount of at least 5 mol per mole of the olefin, and the hydrogen fluoride separated and recovered in step (c) is recycled to step (a). A process according to claim 1 or 2, wherein the olefin has at least 4 carbon atoms, and the reaction temperature and/or the reaction pressure in step (a) are/is adjusted to form esters having a desired kinetic viscosity. A process according to claim 1 or 2, comprising an additional step, before step (b), of distilling the hydrogen fluoride-containing reaction solution from step (a) under reflux with a saturated hydrocarbon which has a higher boiling point than the acyl fluoride and which is inert to the acyl fluoride. A process according to claim 4, wherein the saturated hydrocarbon in the said additional step is used in an amount of 0.5 to 2.0 mol per mole of the olefin. A process according to any one of the preceding claims wherein the saturated hydrocarbon is at least one member selected from hexane, heptane, octane, nonane and decane. A process according to any one of the preceding claims, further comprising: washing the ester-containing solution from step (c) with water; distilling the washed solution; and catalytically hydrogenating the distilled solution, to obtain an ester or esters which have an acid value of 0.02 mgKOH/g or less and which are decolored. A process according to any one of the preceding claims, wherein the alcohol is a monohydric alcohol or an alcohol having at least difunctionality. A process according to any one of the preceding claims, wherein an equivalent ratio of OH group of the alcohol to the olefin is 2 or less.

Inventors: MATSUMOTO SHUNICHI, SATO YOSHIFUMI, SHIOKAWA YOSHIHIRO, TAKEFUMI TADAYOSHI, MATSUMOTO, SHUNICHI, SATO, YOSHIFUMI, SHIOKAWA, YOSHIHIRO, TAKEFUMI, TADAYOSHI, MATSUMOTO, SHUNICHI, C/O MIZUSHIMA WORK OF MITSUB., SATO, YOSHIFUMI, C/O MIZUSHIMA WORK OF MITSUBISHI, SHIOKAWA, YOSHIHIRO, C/O MIZUSHIMA WORK OF MITSUB., TAKEFUMI, TADAYOSHI, C/O MIZUSHIMA WORK OF MITSUB.
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1994
Main classification: C07C51
IPC codes: C07C51, C07C67